Spring中后置处理器与循环依赖的问题

在Spring的bean生命周期中, 我们可以通过实现BeanPostProcessor来对bean初始化前后做操作, 在网上的许多帖子博客中, 不乏可以看见说Spring的AOP是在后置处理器中实现的, 这个理解显然有失偏颇, 很容易误导一般人在后置处理器中对原先的bean进行代理增强再返回替换掉原来的bean引用, 但这极容易引起一个如下error:

二月 13, 2023 9:14:43 下午 org.springframework.context.support.AbstractApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'bean1': Bean with name 'bean1' has been injected into other beans [bean2] in its raw version as part of a circular reference, but has eventually been wrapped. This means that said other beans do not use the final version of the bean. This is often the result of over-eager type matching - consider using 'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.
Exception in thread "main" org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'bean1': Bean with name 'bean1' has been injected into other beans [bean2] in its raw version as part of a circular reference, but has eventually been wrapped. This means that said other beans do not use the final version of the bean. This is often the result of over-eager type matching - consider using 'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:643)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:509)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:326)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:248)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:324)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:901)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:907)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:561)
	at com.yadong.demo01.AnnotationConfigApplicationContextTest.main(AnnotationConfigApplicationContextTest.java:20)

这个错误只有在循环依赖产生时, 并且在后置处理替换掉了原来的bean并重新返回一个新的bean时会产生。

以下是demo示例：

目录结构:

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Bean1.java
package com.yadong.demo01;


import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class Bean1 {

   private String name = "bean1";

   @Autowired
   Bean2 bean2;

   public Bean1(){

   }
   public Bean1(String name){
      this.name = name;
   }


   public Bean2 getBean2() {
      return bean2;
   }

   public void setBean2(Bean2 bean2) {
      this.bean2 = bean2;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   @Override
   public String toString() {
      return "Bean1{" +
            "name='" + name + '\'' +
            ", bean2.name=" + bean2.getName() +
            '}';
   }
}

Bean2.java
package com.yadong.demo01;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;

@Component
public class Bean2 {
   private String name = "bean2";

   @Autowired
   Bean1 bean1;

   public Bean2() {

   }

   public Bean2(String name) {
      this.name = name;
   }

   public Bean1 getBean1() {
      return bean1;
   }

   public void setBean1(Bean1 bean1) {
      this.bean1 = bean1;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   @Override
   public String toString() {
      return "Bean2{" +
            "name='" + name + '\'' +
            ", bean1.name=" + bean1.getName() +
            '}';
   }
}

BeanPostProcessor1.java
package com.yadong.demo01;


import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.stereotype.Component;

@Component
public class BeanPostProcessor1 implements BeanPostProcessor {
   @Override
   public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
      System.out.println("[" + beanName + "]" + "初始化 前处理方法");
      return BeanPostProcessor.super.postProcessBeforeInitialization(bean, beanName);
   }

   @Override
   public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
      System.out.println("[" + beanName + "]" + "初始化 后处理方法");
      return doInsteadBean(bean, beanName);
      //return BeanPostProcessor.super.postProcessAfterInitialization(bean, beanName);
   }

   private Object doInsteadBean(Object bean, String beanName){
      if(beanName.equals("bean1")){
         Bean1 bean1 = new Bean1("替换后的对象bean1");
         Bean2 bean2 = new Bean2("替换后的对象bean1.bean2");
         bean1.setBean2(bean2);
         return bean1;
      }else{
         return bean;
      }
   }
}

AnnotationConfigApplicationContextTest.java
package com.yadong.demo01;

import org.springframework.beans.factory.support.DefaultListableBeanFactory;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

/**
* @author YadongTan
* @Description 循环依赖与bean生命周期测试
*/
public class AnnotationConfigApplicationContextTest {

   public static void main(String[] args) {

      DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
      AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(factory);

      context.register(Config.class);
      context.refresh();

      Bean1 bean1 = context.getBean(Bean1.class);
      System.out.println("bean1 = " + bean1);

      Bean2 bean2 = context.getBean(Bean2.class);
      System.out.println("bean2 = " + bean2);
   }

   @Configuration
   @ComponentScan("com.yadong.demo01")
   protected static class Config{
   }
}

因为1ean1与bean2有循环依赖, 因此创建bean1过程中, 逻辑是:

(bean1)getBean() -> doGetBean() -> createBean() -> doCreateBean() -> getSingleton() -> populateBean() ->… -> (bean2) getBean() -> doGetBean() -> … -> populateBean() ->对Bean2中的bean1字段进行属性填充,。

此时bean1这个bean会被调用getEarlyBeanReference()以提前将对象暴露出来, 在这里方法中, 会调用到WrapIfNecessary(), Spring会对这个bean判断是否需要被AOP增强从而创建代理, 一旦bean1的引用被bean2所持有, 后续在BeanPostProcessor替换掉原先的bean就会导致bean2属性中持有的bean1是旧版本的bean, 也就是没有持有最终版本的bean1(因为你在后置处理中替换掉了原来的bean)。

对于存在循环依赖又想对Bean进行手动代理的, 可以选择实现SmartInstantiationAwareBeanPostProcessor接口的getEarlyBeanReference()方法, 在这里返回代理后的bean。

package com.yadong.demo01;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor;
import org.springframework.beans.factory.config.SmartInstantiationAwareBeanPostProcessor;
import org.springframework.stereotype.Component;

@Component
public class BeanPostProcessor3 implements SmartInstantiationAwareBeanPostProcessor {

   @Override
   public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
      return doInsteadBean(bean, beanName);
   }

   private Object doInsteadBean(Object bean, String beanName){
      if(beanName.equals("bean1")){
         Bean1 bean1 = new Bean1("替换后的对象bean1");
         Bean2 bean2 = new Bean2("替换后的对象bean1.bean2");
         bean1.setBean2(bean2);
         return bean1;
      }else{
         return bean;
      }
   }

}